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Unsaturated fluid flow and volume change behavior of filtered tailings




Aghazamani, Neelufar, author
Scalia, Joseph, IV, advisor
Bareither, Christopher A., committee member
Shackelford, Charles D., committee member
Ham, Jay M., committee member

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As the global demand for minerals continues to increase so does the generation of mine waste. Tremendous volumes of mine waste, viz. tailings and waste rock, are generated and placed in impoundments and piles. Improved methodologies are needed to enhance the sustainability of mine waste management. Tailings are typically discharged as a slurry of finely ground rock within water to a settling facility contained by an embankment. These facilities often necessitate long-term management of inherently weak materials. Tailings dewatered via filtration yields enhanced water stewardship and greater geotechnical stability; filtered tailings are readily amenable to progressive closure and environmental restoration. But, the high cost of tailings filtration, and the potential for acid rock drainage (ARD) due to oxygen ingress and internal unsaturated flow of water have limited the adoption of filtered tailings by the mining industry. The goal of this study is to advance the state of knowledge of filtered tailings. To this end, this dissertation consists of three components; (1) assessing the influence of filtered tailings placement conditions on filtered tailings unsaturated characteristics, (2) assessing excess pore pressure generation during compression of unsaturated filtered tailings, and (3) exploring hydrologic paths to minimize ARD and improve geochemical stability of filtered tailings stacks. Pressure plate, chilled mirror, and shrinkage curve tests were performed to produce soil water characteristic curves (SWCCs) for two precious metal mine tailings with varying initial water contents and dry densities. The resultant SWCCs illustrate that the placement water content and dry density have a significant influence on the unsaturated characteristics of the filtered tailings. Generation of excess pore water pressure was assessed via undrained compression tests. Unsaturated filtered tailings started generating appreciable excess porewater pressure (> 10% of the incrementally applied total vertical stress) when the saturation of the tailings was at the range of 80 to 90%; this appreciable excess pore pressure did not fully dissipate after 24 h. Filtered tailings in this study followed a stress path with void ratios below the critical state line (i.e., dilative during undrained shear) unless placed initially loose and wet. The SWCCs produced were used to model the hydrology of filtered tailings and comingled filtered tailings columns via HYDRUS-2D in example wet, hemiboreal, and dry climates. Results of this study illustrate that for the filtered tailings evaluated in this study, ARD is anticipated to be minimized via varying climate-dependent mechanisms. In the arid climate, filtered tailings functioned as a water balance layer, storing, and releasing precipitation as evaporation not percolation. In the wet climate, filtered tailings became rapidly saturated and maintained a saturated surficial layer preventing inward movement of oxygen and potentially minimizing generation of ARD. In the hemiboreal and wet climates percolation was minimal due to the low hydraulic conductivity of the filtered tailings, and inclusion of a commingled capillary barrier layer further reduced percolation (further reducing the potential flux of ARD). Results from this study illustrate the potential efficacy of filtered tailings to maintain both geotechnical stability and limit ARD.


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Embargo Expires: 05/24/2024



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